(397g) Simulation of Worm Like Micelle Assisted Assembly of Linear Nanostructures

In surfactant assisted synthesis of nanoparticles, morphologies of the micelles play the role of guiding evolution of shape of nanoparticles. Depending upon the concentration of surfactant and counter-ions, micelles exhibit a range of morphologies such as spheres, cylinders, worm like micelles (WLM), etc. Formation of silver halide nanorods of high aspect ratio, assisted by worm like micellar phase at room temperature, have been demonstrated by Gupta et. al., (2012) (Colloids and Surfaces A 393 (2012) 73–80) recently. In the present work we propose a mechanism for formation of these nanorods.

The WLM phase is characterized by high degree of entanglement amongst the worm-like chains; resulting in an increase in the viscosity by over four orders of magnitude (as compared with an aqueous phase). This causes a substantial reduction of the particle diffusivity which in turn reduces frequency of collision between the (primary) spherical particles. The attractive Van der Waals interaction between the particles and the chains therefore leads to adsorption of the particles on backbone of WLM, resulting in decoration of the WLM chains with spherical nanoparticles. During their relaxation by reptation the micelles bring particles closer to each other and promote the formation of linear assemblies of the particles. This mechanism is simulated using a frame work of slithering snake dynamics for WLM. In this study the worm like micelles are represented by flexible polymers of fixed contour length. Further, a rule based intermicellar particle exchange protocol is formulated and simulated on a periodic lattice. Simulations reveal that the particles start accumulating slowly on few of the micellar backbones; towards the end the fraction of micelles carrying no particles increases from 15% to nearly 70% which is typical behavior observed in coagulation processes. The particulate masses accumulated on the WLMs are then converted to their respective lengths and diameters. Further simulations like effect of precursor concentration and surfactant volume fraction are being done to gain more understanding of the system.